As an example of conventional transmitter circuits, a transmitter circuit that generates a transmission signal by using a quadrature modulation method is known. Since such transmitter circuits using the quadrature modulation method are widely known, the description thereof will be omitted. In addition, as an example of conventional transmitter circuits that operate more efficiently than the circuits using the quadrature modulation method, an EER modulation circuit using an EER (Envelope Elimination and Restoration) modulation method is known. In the EER modulation method, an input signal is divided into a phase-component signal and an amplitude-component signal. First, an oscillation signal generated by an oscillator is multiplied by the phase-component signal, whereby a phase-modulated signal having a constant amplitude is generated. Next, by using a saturation amplifier, the amplitude-component signal is amplified and superimposed onto the phase-modulated signal, whereby a transmission signal is generated.
In the EER modulation method, there is a feature that a saturation amplifier is used when the amplitude component is superimposed onto the phase-modulated signal. Since the saturation amplifier is operated in a saturation region, a transmission signal can be generated with high power efficiency.
However, in the EER modulation method, when the output level of a transmission signal is low, the amplifier operates outside the saturation region, whereby power efficiency decreases, or the proportion of power consumption of an amplitude modulation section to power consumption of the entirety of the transmitter becomes large, whereby power efficiency decreases. Therefore, conventionally, there has been proposed a transmitter circuit that linearly operates the amplifier, using the EER modulation method for high level of output and using the quadrature modulation method for low level of output, thereby improving power efficiency. For example, Patent Literature 1 discloses a transmitter circuit 600 shown in FIG. 7. Hereinafter, the configuration and the operation of the transmitter circuit 600 will be described.
An in-phase component signal (I-signal) and a quadrature component signal (Q-signal), which are signals for the quadrature modulation method, are inputted from a baseband section 601 to an interface section 602. An Rθ conversion section 604 in the interface section 602 switches a modulation method between the quadrature modulation method and the EER modulation method, based on an AGC control signal from the baseband section 601. In the quadrature modulation method, the Rθ conversion section 604 outputs the I-signal and the Q-signal as they are, without performing signal processing, and in the EER modulation method, the Rθ conversion section 604 performs processing of converting the I-signal and the Q-signal into an amplitude-component signal and a phase-component signal (Rθ conversion processing). The Rθ conversion processing is performed by extraction of phase information by a limiter, and envelope detection.
In the quadrature modulation method, the I-signal is inputted to a DAC 605, and in the EER method, the phase-component signal is input to the DAC 605. In addition, in the quadrature modulation method, the Q-signal is inputted to a DAC 606, and in the EER method, the amplitude-component signal is inputted to the DAC 606. An output from the DAC 605 is inputted to a mixer 621 via a baseband filter 625.
In the quadrature modulation method, a switch 607 connects the output of the DAC 606 to a Q-component baseband filter 608 in an RF-IC 603, and in the EER modulation method, the switch 607 connects the output of the DAC 606 to an amplitude modulation circuit 609.
In the quadrature modulation method, a switch 610 connects the sum of the I-signal and the Q-signal to an AGC amplifier 611, and in the EER modulation method, the switch 610 connects only the phase-component signal to the AGC amplifier 611. In the quadrature modulation method, a switch 612 connects the output of the AGC amplifier 611 to an output buffer 613, thereby transmitting an output from the AGC amplifier 611, to an front end not via a power amplifier 614, and in the EER modulation method, the switch 612 connects the output of the AGC amplifier 611 to the power amplifier 614, thereby amplifying an output from the AGC amplifier 611.
In the quadrature modulation, an input signal is converted into the I-signal and the Q signal. An oscillation signal generated by an oscillator 620 is distributed into two lines by a phase shifter 623. One of the two signals is outputted to the mixer 621 without shifting the phase of the signal, and the mixer 621 multiplies the signal by the I-signal outputted from the baseband filter 625. The other one of the two signals is outputted to a mixer 622 after the phase of the signal is shifted, and the mixer 622 multiplies the signal by the Q-signal outputted from the baseband filter 608. Thereafter, the signals which have been respectively multiplied by the I-signal and the Q-signal are synthesized by the adder 624, whereby a modulated wave based on the quadrature modulation method is obtained. In the EER modulation method, an input signal is converted into the amplitude-component signal and the phase-component signal. First, an oscillation signal generated by the oscillator 620 is multiplied by the phase-component signal, whereby a phase-modulated signal is generated. Thereafter, the amplitude-component signal is amplified and superimposed onto the phase-modulated signal by the power amplifier 614, whereby a modulated wave based on the EER modulation method is obtained. That is, the transmitter circuit switches the modulation method such that if the voltage level of a signal is smaller than a predetermined value, the quadrature modulation is performed, and if the voltage level is larger than the predetermined value, the EER modulation is performed. In this way, the conventional transmitter circuit uses the quadrature modulation method and the EER modulation method in a combined manner, thereby realizing reduction in the power consumption.